It's just handled automatically, like regular captains: if they're a melee-oriented Kohan, they'll be aggressive; if they're a spellcaster or archer, they'll hang back.

All you lose when a Kohan dies is their experience. You don't have to pay to resurrect them, like you did in the first two games; and AFAIK there's no way to permanently lose them (i.e., no stealing amulets because there are no amulets in the first place).